Cheap ball bearings.
If you look around long enough, eventually you can find a cheap source of almost anything. It turns that in the case of ball bearings, the secret word is "slingshot ammunition". These may not be very high-precision, but they are many times cheaper than those sold for actual use in actual bearings.Source:CabelasContributor:Theodore GrayAcquired:26 September, 2003Price: $15/400Size: 3/8"Purity: 99%